U.S. Pat. No. 4,992,890, for an invention by Pinard, Girzon and Warner, entitled "System for Plotting and Scanning Graphic Images," describes a system for plotting and scanning graphic images and is incorporated herein by reference. The ColorSetter 2000 and ColorSetter 4000 products, made by the Optronics Division of Intergraph Corporation, Chelmsford, Mass., use technology disclosed in this U.S. patent. (ColorSetter is a registered trademark of Intergraph Corporation.)
The system described in U.S. Pat. No. 4,992,890 makes it possible to scan or plot an n-pixel wide swath using only one light-beam source. FIG. 1A of the present application shows the arrangement used in this system. An acousto-optical (A/O) crystal serves as the deflection means 10, which deflects the light beam 18 from an undeflected path, denoted in FIG. 1A as P.sub.0. The degree that the A/O crystal deflects the beam 18 is a function of the frequency of a vibratory waveform, which is created by a transducer 14--typically a piezo-crystal--which converts electrical signals from a radio-frequency signal generator 12.
The signal generator 12 delivers a set of up to n frequencies, each of which causes one of the light beam paths P.sub.1, P.sub.2, P.sub.3 . . . P.sub.n. Whether a portion of the beam 18 follows one of these paths depends on whether a vibratory waveform at a certain frequency is applied to the A/O crystal. The strength of the sub-beam--i.e., deflected beam portion--that follows that path is a function of the amplitude of the vibratory waveform at the frequency associated with that path. When no vibratory stimulus is applied, all of the original beam 18 follows path P.sub.0 and is blocked by a mask 24 from reaching the target media (substrate) 26, which may be a film that is to be exposed so as to create an image thereon, or which may contain an image to be scanned. If the vibratory stimulus has two different frequency components, the original beam 18 is split into two separate first-order sub-beams. Non-first-order sub-beams and heterodyned beams are also produced, but these extraneous beams are also blocked by the mask 24.
In one of the embodiments disclosed in U.S. Pat. No. 4,992,890, the substrate 26 is mounted on a rotating drum. Plotting information for a swath is broken up into n portions, one for each of the N sub-beam paths (P.sub.1 -P.sub.N). In the example discussed in U.S. Pat. No. 4,992,890, up to eight different vibratory signals are created, and two sub-beams expose the substrate 26 at any one instant. First, sub-beams along paths P.sub.1 and P.sub.5 are directed towards the substrate. Then sub-beams along paths P.sub.2 and P.sub.6, then sub-beams along paths P.sub.3 and P.sub.7, and finally sub-beams along paths P.sub.4 and P.sub.8 are directed towards the substrate. While all eight of these sub-beams have the opportunity to expose the target substrate 26, the drum on which the substrate is mounted is rotating, and, after all eight of these sub-beams has exposed the substrate, the drum has rotated so that the circumferential surface of the drum has moved the distance of one pixel. The sequence of sub-beams is then repeated: first, paths P.sub.1 and P.sub.5 are used, then paths P.sub.2 and P.sub.6, etc. In effect, two sub-beams together scan the width of the swath, with each of the two sub-beams scanning four pixels during each scan.
The angle of deflection caused by the A/O crystal 10 is tilted so that the rotation of the target drum on which the substrate 26 is mounted causes the eight pixels formed by each of the sub-beams to line up in the manner shown in FIG. 1B. (Of course, depending on the image to be formed on the substrate, less than all--or perhaps none--of the eight pixels shown in FIG. 1B may be exposed by the sub-beams.) The set of eight pixels shown in FIG. 1B is repeatedly formed one below the other so as to cover a swath each time the drum rotates. Since the scanning head is mounted on a carriage that constantly moves in a direction parallel to the drum's axis of rotation, while the drum rotates, swaths are formed next to each other so as to cover the surface of the substrate that is to be exposed. This series of swaths covering the substrate may be considered a single helical swath around the surface of the drum. The system may be adapted so that any number of sub-beams may be used.
In the prior-art ColorSetter plotters, the beam's shaping assembly 22, the A/O crystal 10, the piezo-crystal 14 and the mask 24 are mounted on the moving optics carriage, which is moved in a direction parallel to the drum's axis of rotation by a rotating lead screw. These plotters were typically used to create separates on a transparent substrate. Such separates are used to form printing plates used in printing multicolored images. Typically four separates are made, one for each of the printing plates that will apply cyan, magenta, yellow and black ink.
U.S. Pat. No. 4,985,779, for an invention by Gall, entitled "Improved Method and Apparatus for Generating Halftone Images," discloses a system for generating halftone images and is also incorporated herein by reference. It uses coordinate transformation and look-up tables to screen the image information. The screened image information is sent to radio-frequency signal generators, which generate signals for the acousto-optical modulators in the scanning head. The microdots referred to in U.S. Pat. No. 4,985,779 are referred to as pixels herein. A plurality of pixels, or microdots, are used to form a halftone dot.